Orbital polaron in double exchange ferromagnets

TL;DR

This paper studies the spectral properties of a two-orbital Hubbard model with double exchange, revealing an orbital polaron and emphasizing the role of double hopping in ferromagnetic materials like manganites.

Contribution

It introduces the concept of an orbital polaron and demonstrates the significance of double hopping in the spectral features of double exchange ferromagnets.

Findings

Spectral function shows a three-peak structure.

Existence of a ferromagnetic phase near n=1 due to double exchange.

Identification of an orbital polaron at the Fermi energy.

Abstract

We investigate the spectral properties of the two-orbital Hubbard model, including the double hopping term, by means of the dynamical mean field method. This Hamiltonian describes materials in which ferromagnetism is realized by the double exchange mechanism, like for instance manganites, nickelates or diluted magnetic semiconductors. The spectral function of the unoccupied states is characterized by a specific equidistant three peak structure. We emphasize the importance of the double hopping term on the spectral properties. We show the existence of a ferromagnetic phase due to electron doping near n=1 by the double exchange mechanism. A quasi-particle excitation at the Fermi energy is found that we attribute to what we will call an orbital polaron. We derive an effective spin-pseudospin Hamiltonian for the two-orbital double exchange model at n=1 filling to explain the existence and dynamics of this quasi-particle.